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Once scientists began studying the impact of global warming on everything
from tourism to asthma, it was only a matter of time before they got around
to sex. Now two biologists at Spain's Higher Council for Scientific Research
(CSIC) have done just that, at least when it comes to fish.

You may have missed it in biology class, but in some finned species, like the
Atlantic silverside  as well as in many reptiles  sex is determined
not by genetics but by temperature: the undifferentiated embryo develops
testes or ovaries on the basis of whichever option conveys evolutionary
advantages for that particular environment. Now, in a study published in the
July 30 edition of the scientific journal Public Library of Science, Natalia Ospina-Alvarez and Francesc Piferrer have gone a little further
in explaining how that mechanism works. In laboratory tests, they have
demonstrated that higher water temperatures result in more male fish.

"We found that in fish that do have temperature-dependent sex determination
[TSD], a rise in water temperature of just 1.5 degrees Celsius can change the male-to-female ratio from 1:1 to 3:1," says Piferrer, the study's co-author. In
especially sensitive fish, a greater increase can throw the balance even
more out of whack. Ospina-Alvarez and Piferrer have found that in the
South American pejerrey, for example, an increase of 4 degrees Celsius can result in a
population that is 98% male.

What makes these findings especially troubling, of course, is that the
International Panel on Climate Change (IPCC) predicts that ocean-water temperatures
are likely to rise by 1.5 degrees over the course of this century  and they may even go
up a few degrees more. "If climate change really does result in a rise of
4 degrees, which is the maximum the IPCC predicts, and if species can't adapt in
time or migrate, then in the most sensitive cases of TSD, we're looking at
extinction," says Piferrer.

Most research into fish sex determination has been done in the lab (for
obvious reasons), but the pejerrey is one of the few species that scientists
have been able to study in the field. And those studies have revealed that
already, its proportion of males to females is skewed. "It
could be because of chemical pollution or it could be because of climate
change. We don't know," cautions Piferrer. "But the field data matches our
predictions."

At this stage, it is hard to tell what these results bode for already
declining fish populations around the world. Of the estimated 33,000
piscatorial species, only 5,000 have had their sex-determination mechanism
affirmed. But the study by the two CSIC scientists also suggests that the
percentage of TSD fish is lower than previously believed. In tests of 59
species believed to be reproductively sensitive to temperature, only 40
proved to be true TSDs.

That would be good news in this grim era of climate change if it weren't for
one factor: even genotypic sex determination can be affected by anomalous
conditions, including anomalous temperature. "Basically, if you freeze it or
cook it enough," says Piferrer, "you can get whatever sex you want."